I have no idea where you read this, but it's not right. The immune system doesn't know anything about the brand of a vaccine. It remembers proteins and how to make antibodies to bind to proteins. Pretty much all vaccines are based on the exact RNA sequence of the spike protein from the first published COVID sequence. Immune systems can and do adapt to mutations in the spike protein by generating slightly different antibodies. It's how we fight colds and the flu, since these are constantly changing.

The main difference between the vaccine brands is the delivery mechanism. There were concerns in the beginning about mixing and matching; those were concerns about dosage, which was carefully determined in trials. Clearly a totally different mechanism screws with dosage. But nowadays I don't see many concerns about mixing and matching, particularly if just the booster differs from the (e.g. two first doses).

What I mean is you can take two different vaccines, but it won't result in two different antibody responses. If you take a less effective vaccine like the J&J, doubling up on the Pfizer vaccine isn't expected to 'upgrade' your antibody response, though it can still strengthen immune response. If spike proteins change the immune response may change, but in this situation all vaccine are based on the same spike protein.

The science is always developing, but mixing and matching vaccines tends to lead to more side effects according to at least one study. From what I remember personally in the beginning doctors were recommending people keep the same manufacturer but that isn't considered necessary anymore

https://www.verywellhealth.com/side-effects-mix-and-match-co...

Different manufacturers use different compounds at different concentrations. What do you mean "you can't switch"?

Plenty of people are getting moderna boosters after the first two being pfizer

What I meant is you won't develop separate antibody responses to each individual vaccine. You can mix and match the initial vaccine and boosters, and it'll strengthen your immune response. You just can't switch and expect the same response as someone taking that particular vaccine for the first time

>separate antibody responses

Of course not, they use the same mRNA. There's no reason to suspect they would in the first place.

The difference in response in the degree/intensity of response to the detected antigen. This is due to the concentration of mRNA in the vaccine (and therefore the concentration of antigen produced by the vaccine)

Moderna creates a greater response than Pfizer due to concentration (100mcg vs 30mcg). I don't think there's a reason to believe that a moderna booster on top of a Pfizer initial vaccination would not increase the degree of response when an actual infection is detected. Immunology is not my field however so my knowledge is a bit limited.

Yes you can, it is possible in most country, even sometimes recommended.

For now, in most of the western world, only mRNA boosters are recommended, especially if your first dose(s) are of another vaccine. Moderna and Pfizer/BioNTech shots can be mixed, Pfizer/BioNTech is recommended if you are a young man (risks of myocardite), and for others, there is a slight preference for using the other one.